1. Field of the Invention
The present invention relates to a hollow electrode for use in electroslag remelting and a process of producing an alloy using the same. More specifically, the invention relates to a process of producing a retaining ring material made of non-magnetic radical iron alloy and used for a turbine generator, a semi-high-speed steel working roll for use in cold rolling, a process of producing a hot-rolling forged-steel working roll material which is used for steel rolling and is excellent in heat, impact and crack resistant properties, and is wear resistant, a process of producing a radical Ni--Fe heat resistant alloy ingot by means of electroslag remelting, a process of producing a radical iron heat resistant alloy for use in a gas turbine and a superconductive generator member and a process of producing a high pressure-low pressure single cylinder turbine rotor for use as a turbine rotor shaft of a generator.
2. Prior Art
In order to increase productivity, the adoption of large-scale production facilities and the enforcement of rigid operating conditions is in progress in power generation, chemical, iron and steel industries and so forth. It is now common that forged materials for use in those facilities are produced from ESR ingots through the electroslag remelting method (hereinafter called "ESR method") so as to ensure safety in operation.
The ESR method is intended to obtain ingots having a smooth surface and good internal properties by remelting an electrode with heat resulting from supplying power from the solid electrode, causing the molten electrode (i.e., a consumable electrode) to drop on a slag, and directionally solidifying the molten metal pool in a mold. In order to obtain such an ingot of good quality, it is necessary to supervise the molten metal pool while keeping the slag temperature at a suitable level. In other words, the determination of the ESR conditions is dependent on factors such as electrode feed velocity, voltage, current, the depth of a slag bath, slag composition, a fill ratio (electrode diameter/mold diameter) and the like.
Since carbon or low alloy steel is relatively less sensitive to macro freckle or streak segregation, the segregation poses only a few problems when a small ingot is produced. On the other hand, a high alloy containing a large amount of elements such as Ni, Cr, Mn and the like, such as a radical Ni or Co super alloy, is highly sensitive to segregation. Consequently, segregation occurs even when a relative small ingot is produced and this poses still another problem in that products exhibiting good performance are not manufactured.
The problems described above arise in following examples.
A retaining ring material made of a non-magnetic iron radical alloy and used for a turbine generator is often produced from an ESR ingot through the electroslag remelting method in the attempt to improve its internal properties.
The retaining ring material intended for the present invention is what has been standardized and known as ASTM A289 Classes B, C. Since such a material contains a large amount of Mn and Cr, even when the ESR method is employed and the aforesaid factors are controlled, macro freckle or streak segregation tends to appear in an ingot. Consequently, there arise cases where products exhibiting satisfactory performance are unavailable.
Recently, cold-rolling working rolls for actual use increasingly need to meet severe quality requirements as attempts are being made to increase efficiency of the rolling process. In order for such a working roll to bear continuous severe heavy-load, high-speed operating conditions, it is important to improve its fail-safe and wear resistant properties. Attention has also-been directed to cold-rolling working roll material of semi-high-speed steel highly resistant to wear and injury resulting from rolling, the material containing a carbide forming element other than Cr and causing a harder carbide to be separated out. As this semi-high-speed steel working roll material has a strong tendency for segregation, a special melting method called an electroslag remelting method has been employed to reduce the segregation.
The most important factor by which the soundness of the use layer of a semi-high-speed roll is impaired results from the appearance of streak segregation (inverted V segregation). If such segregation appears in a position close to the surface, the effective use diameter of a roll to be manufactured is narrowed. Moreover, difficulties in roll production are maximized as the risk of destruction at the time of hardening increases.
However, the problem is that though ESR is applied to the manufacture of the roll while the aforesaid factor is put under control, it is still difficult to eliminate the streak segregation sufficiently when the segregation tendency is great due to the composition. In other words, the small effective diameter of the roll is likely and thus profitability is reduced.
Further, radical Ni--Fe heat resistant alloy (represented by Inconel (trade name) 718 and 706 alloys) ingots may normally be obtained through the electroslag remelting method so as to improve their internal properties. The ESR method is effectively utilized-particularly for large-sized ingots to prevent segregation.
Heat resistant alloy like radical Ni--Fe alloy is very sensitive to segregation as it contains a large amount of alloy elements. Even when a relatively small ingot which generally does not generate segregation as compared with a large one is produced, the ESR method is applied thereto and adequate control is exerted as previously noted. Notwithstanding, this macro freckle or streak segregation tends to appear on the ESR ingot and this still poses a problem in that products exhibiting good performance remain unavailable.
Moreover, an ESR ingot of Inconel alloy 718 among radical Ni--Fe heat resistant alloys has a poor surface and tends to cause forging fracture. For this reason, the surface of the ingot is machined to smooth it before being forged. However, another problem of deteriorating hot-rolling workability due to removal of the shell arises as the dense layer in the surface of the ingot is removed. In addition, there arises still another problem of lowering the ingot yielding rate as the high-quality portion of the ingot is not utilizable.
Radical iron heat resistant alloys as indicated by the standard Nos. JIS G4311.about.4312 SUH660, which offer high-temperature strength and excellent wear resistance, are used for gas turbine and jet engine members. As such alloys are capable of further offering greater strength, excellent toughness and stable non-magnetic properties at cryogenic temperatures. They are also used for superconductive generator members.
The material needs to meet severe user requirements and to provide greater durability in practical use as previously noted. The mechanical properties of a radical iron alloy is also largely affected by the presence of a brittle deposit phase or a nonmetal inclusion. Consequently, a melt-refining process is required to minimize impurities in addition to rendering alloy design adequate. A special melting method called an electroslag remelting method has been employed for this purpose.
The most important factor by which the soundness of the quality of a SUH660 radical iron alloy is impaired results from the appearance of streak segregation and the segregation increases in percentage as the diameter of an ingot increases. However, the radical iron alloy is highly sensitive to segregation as it contains a large amount of alloy elements and even though ESR is applied to the manufacture of the ingot while the aforesaid factor is put under control during this ESR operation, it is still difficult to evade the macro segregation sufficiently.
As one of the turbines of generators, a high pressure-low pressure single cylinder turbine incorporating the high-pressure portion up to the low-pressure portion is well known and a high pressure-low pressure single cylinder turbine rotor is used for such a turbine.
The turbine rotor is usually exposed to high-temperature, high- and low-pressure steam and consequently material forming the rotor should be provided with not only satisfactory high-temperature creep strength but also excellent low-temperature toughness. However, only one kind of material can adequately satisfy these requirements. Accordingly the high pressure-low pressure single cylinder turbine rotor of the sort that has been proposed so far is made to suit the operating conditions in a manner that the portion corresponding to high-medium pressure is made of Cr--Mo--V steel offering good high-temperature creep properties, whereas what corresponds to low pressure is made of Ni--Cr--Mo--V steel also offering excellent low-temperature toughness. There may be various methods of manufacturing such a composite turbine rotor but industrially the electroslag remelting method is considered most suitable. In this respect, Japanese Examined Patent Applications No. 4254/1977 and No. 14842/1981, Japanese Unexamined Patent Publication No. 23367/1981, No. 105502/1982 and No 135536/1985 disclose processes of manufacturing such a composite turbine rotor.
If, however, materials are melted to manufacture a composite turbine rotor through the ESR method, a wide transition area would be formed between portions which are different in composition as different ingredients on both sides mix well and this poses a problem in that desired properties are not obtained. Therefore, it still remains industrially unfeasible to produce composite turbine rotors through the ESR method.